U.S. patent number 5,435,878 [Application Number 07/678,255] was granted by the patent office on 1995-07-25 for method for welding polycarbonate by high frequency.
This patent grant is currently assigned to Gemplus S.C.A.. Invention is credited to Jean-Claude Delmar, Christian Schmuckle.
United States Patent |
5,435,878 |
Delmar , et al. |
July 25, 1995 |
Method for welding polycarbonate by high frequency
Abstract
The invention relates to a method for welding thermoweldable
sheets of which at least one is made of polycarbonate. The method
is characterized in that the sheets to be welded are separated from
the metal electrodes (4,5) of the high frequency press by means of
a glass plate (2,3). Application to the fabrication of high density
magnetic cards.
Inventors: |
Delmar; Jean-Claude (Couches,
FR), Schmuckle; Christian (Chalon S/Saone,
FR) |
Assignee: |
Gemplus S.C.A. (Gemenos,
FR)
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Family
ID: |
9371447 |
Appl.
No.: |
07/678,255 |
Filed: |
April 18, 1991 |
Foreign Application Priority Data
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Oct 18, 1988 [FR] |
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88 14222 |
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Current U.S.
Class: |
156/272.2;
156/273.3; 156/274.4; 156/272.4; 156/289 |
Current CPC
Class: |
B29C
66/45 (20130101); B29C 66/80 (20130101); B29C
65/04 (20130101); B29C 66/0342 (20130101); B29C
66/0346 (20130101); B29C 66/1122 (20130101); B29C
59/02 (20130101); B32B 37/26 (20130101); B29C
66/81457 (20130101); B29C 66/8322 (20130101); B29C
66/71 (20130101); B29C 66/723 (20130101); B29C
66/8122 (20130101); B29C 66/71 (20130101); B29K
2069/00 (20130101); B29C 66/8122 (20130101); B29K
2909/08 (20130101); B29C 66/919 (20130101); B32B
2429/00 (20130101); B29K 2909/08 (20130101); B29C
66/0242 (20130101); B29C 66/929 (20130101); B32B
2310/12 (20130101); B29K 2069/00 (20130101); B29L
2009/00 (20130101); B32B 2425/00 (20130101); B29C
66/949 (20130101); B29C 66/71 (20130101); B29K
2027/06 (20130101) |
Current International
Class: |
B29C
65/04 (20060101); B29C 65/00 (20060101); B29C
59/02 (20060101); B32B 031/00 () |
Field of
Search: |
;156/272.2,273.3,274.4,272.4,289 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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2120070 |
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Aug 1972 |
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FR |
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2312629 |
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Jan 1974 |
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DE |
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Other References
"Dielectric Heating," Encylopedia of Polymer Science &
Technology vol. 5, 1986. .
Readdy, A. F., "Plastics Fabrication by Ultraviolet, Infrared,
Induction, Dielectric, & Microwave Radiation Methods,"
Plastics, Picatinny Arsenal, Dover, N.J. 1972 p. 57. .
Japanese Patent Gazette, Semaine D05, 11 mars 1981, section CH,
classe A, p. 2, resume No. 06553, Derwent Publications Ltd.
Londres, GB & JP-A-55 150321 (Sumitomo Bakelite K.K.) 22 Nov.
1980 voir resume (cite dans 1a demande). .
Japanese Patent Gazette, semaine 8732, 23 Sep. 1987, section CH,
classe A, p. 26, resume No. 87-225838/32, Derwent Publications
Ltd., Londres, GB & JP-A-62 151 323 (Nippon Shigyo K.K.) 6 Jul.
1987..
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Primary Examiner: Barry; Chester T.
Attorney, Agent or Firm: Oblon, Spivak, McClelland, Maier
& Neustadt
Claims
We claim:
1. A method for welding heat-weldable sheets, wherein at least one
of the sheets is made of polycarbonate, comprising the steps
of:
(1) superimposing said heat-weldable sheets to form a sandwich;
(2) introducing said sandwich between two metal electrodes of a
high frequency press;
(3) pressing said sandwich in the high frequency press and applying
a frequency of 25-30 MHz, wherein said pressing and applying is
through two glass plates each cooperating with a metal electrode of
said high frequency press to heat-weld said sheets, and
(4) cooling said heat-weldable sheets under pressure in a press
which is different than said high frequency press.
2. The method of claim 1, wherein said sandwich is pre-heated to a
temperature of above 100.degree. C. before said pressing step.
3. The method of claim 2, wherein said sandwich is pre-heated to a
temperature of about 150.degree. C.
4. The method of claim 1, wherein said pressing is at a pressure of
100 to 1000 N/cm.sup.2.
5. The method of claim 1, wherein said glass plates have a
thickness of 2-5 mm.
6. The method of claim 1, wherein said frequency is 27.12 MHz.
7. The method of claim 1, wherein at least one of said
heat-weldable sheets supports a magnetic layer.
8. The method of claim 1, wherein said heat-weldable sheets consist
of polycarbonate sheets.
9. The method of claim 8, wherein said heat-weldable sheets consist
of 2-10 polycarbonate sheets, each polycarbonate sheet having a
thickness between 25-200 .mu.m.
10. A method for welding heat-weldable sheets, wherein at least one
of the sheets is made of polycarbonate, comprising the steps
of:
(1) superimposing said heat-weldable sheets to form a sandwich;
(2) introducing said sandwich between two metal electrodes of a
high frequency press;
(3) positioning said sandwich between two glass plates each
cooperating with a metal electrode of said high frequency
press;
(4) positioning a sheet of flexible material between said metal
electrodes and said glass plates;
(5) pressing said sandwich in the high frequency press and applying
a frequency of 25-30 MHz, wherein said pressing and applying is
through said glass plates and said flexible material to heat-weld
said sheets; and
(6) cooling said heat-welded sheets under pressure in a press which
is different than said high frequency press.
11. The method of claim 10, wherein said flexible material
comprises cardboard or polytetrafluoroethylene.
12. The method of claim 10, wherein said sandwich is pre-heated to
a temperature of above 100.degree. C. before said pressing
step.
13. The method of claim 12, wherein said sandwich is pre-heated to
a temperature of about 150.degree. C.
14. The method of claim 10, wherein said pressing is at a pressure
of 100 to 1000 N/cm.sup.2.
15. The method of claim 10, wherein said glass plate has a
thickness of 2-5 mm.
16. The method of claim 10, wherein said frequency is 27.12
MHz.
17. The method of claim 10, wherein at least one of said
heat-weldable sheets supports a magnetic layer.
18. The method of claim 10, wherein said heat-weldable sheets
consist of polycarbonate sheets.
19. The method of claim 10, wherein said heat-weldable sheets
consist of 2-10 polycarbonate sheets, each polycarbonate sheet
having a thickness between 25-200 .mu.m.
Description
This application is a continuation of PCT/FR89/00541, filed Oct.
18, 1989, now abandoned.
The present invention relates to a method for the high-frequency
welding of sheets to one another, at least one of these sheets
being made of polycarbonate.
There are known methods in the prior art for welding polycarbonate
sheets together. The method most commonly used consists in
combining the effects of heat and pressure. In this method, the
sandwich formed by the different sheets to be welded is introduced
into a heat press. The temperature should be higher than the glass
transition temperature of the plastic material and this temperature
is generally between 160.degree. C. and 180.degree. C. The pressure
applied varies between 100 and 1000 N/cm.sup.2. Owing to the time
taken for the temperature build-up and cooling under pressure, the
time needed to weld these polycarbonate sheets is generally from 40
to 45 minutes.
In the same way, there is a known method, in the prior art, for
using high frequency to weld certain materials.
High-frequency welding results from the heating of the material to
be welded by the application of a high-amplitude high-frequency AC
voltage. To be weldable by high frequency, a plastic material
should be permanently dipolar and should be constituted by mobile
molecular chains capable of oscillating, with their mobility
depending on numerous parameters, notably on the conditions of
manufacture, internal and external plasticization, temperature,
frequency and amplitude of the high frequency. Thus, PVC is a
particularly appropriate material for being welded by high
frequency. This welding is done between two metal electrodes kept
under pressure during the application of the high frequency. Now,
trials have demonstrated that such a technique cannot be applied to
polycarbonate, one reason for this being that the temperature for
starting the reaction (about 100.degree. C. or more) is too
high.
Thus the U.S. Pat. No. 4,678,713 shows that polycarbonate belongs
to this category of materials that cannot be welded by high
frequency. In fact, according to this patent, the welding of
polycarbonate implies the use of an adhesive layer formed by
polymers containing carbon monoxide.
Similarly, the application JP-A-55 150 321 also shows that
polycarbonate is among the materials, having a low dielectric
dissipation factor, that cannot be dielectrically welded by
themselves. Hence, a layer of a plastic resin with a high
dielectric dissipation factor is added to these materials, making
said materials with a low dissipation factor capable of being
welded by high frequency.
The document FR-A-2 120 070 describes a method for the welding,
notably, of two polycarbonate sheets, several millimeters thick,
that have been shaped beforehand by undergoing heating at a
temperature of the order of 150.degree. C. This method consists in
joining these two shaped sheets by inserting them between two rigid
glass plates that have complementary facing surfaces, and in then
heating the whole in an autoclave at a temperature of 149.degree.
C. and a pressure of several kilograms per cm.sup.2. This method is
not suitable for polycarbonate sheets having a thickness of the
order of one-tenth of a millimeter.
An object of the invention, therefore, is a method for the welding
of polycarbonate sheets in a period of time that is appreciably
shorter than that obtained by the prior art methods.
Yet another object of the present invention is to enable the
high-frequency welding of polycarbonate sheets to one another
without the use of additional adhesive-forming sheets.
Other objects of the present invention shall appear in the
following more detailed description.
The invention relates to a method for the welding of at least two
polycarbonate sheets, wherein:
the sheets that are to be welded to one another are superimposed so
as to form a sandwich,
the sandwich thus made is introduced between the two glass
plates,
characterized in that
each of said glass plates cooperates with a metal electrode of a
press of the high-frequency heating type,
the press is put under pressure,
electric signals are applied to said metal electrodes, the
frequency of which is appropriate to exciting the molecular bonds
of the materials of said polycarbonate sheets, through said glass
plates, and
cooling is done.
The following description is made with reference to the drawings,
Wherein:
FIG. 1 shows a schematic view of the general device for
implementing the method according to the invention.
FIG. 2 shows a schematic view of an embodiment of a device for
implementing the method, according to the invention, for the
manufacture of high-density magnetic cards.
We shall now refer to FIG. 1 showing a schematic view of the
general device for implementing the method according to the present
invention.
First of all, the sheets to be welded together are superimposed to
form a "sandwich" (1). This sandwich (1) may be formed by
polycarbonate sheets alone or else by polycarbonate sheets mixed
with sheets of another material such as PVC for example.
The sandwich thus made is then preferably placed between two glass
plates (2), (3), but actually it is enough for one of the faces of
the sandwich to be in contact with a glass plate. This glass should
have certain characteristics. It should be capable of withstanding
heat shocks as well as compression. To this effect, tempered plate
glass or a glass ceramic of the vision of Corning.TM. type may be
used.
The thickness of the plates varies as a function of the thickness
of the layers to be joined together, and may be of the order of 2
to 5 mm.
The assembly formed by the polycarbonate sandwich in between the
two glass plates is then placed between the jaws (4), (5) of the
press constituted by two metal blocks. At the same time, the two
metal blocks play the role of the electrodes for applying the high
frequency.
In a preferred embodiment of the present invention, a sheet (6) is
provided, made of a flexible material that does not react to high
frequency, between the glass plates and the metal forming each
electrode, to absorb the defects due to poor planeity of the
electrode surface or of the surface of the glass plates, in contact
with the metal electrode. As a matter of fact, owing to the
pressure exerted, it is absolutely indispensable that no bending
stress should be exerted on the glass plates when they are under
pressure. For example, cardboard or Teflon.TM. may be used.
Then, when the assembly is thus arranged, the pressure (between 100
and 1000 N/cm.sup.2) is established and the welding is done by the
application of high frequency. The high frequency should have a
value such that it can excite the molecular bonds of the
polycarbonate and is generally between 25 and 30 MHz, and
preferably 27.12 MHz. Under these conditions, the time taken to
apply the high frequency may be of the order of 5 s to 20 s and is
generally less than 10 s. After that, it is necessary to wait for
the assembly to cool before separating the polycarbonate sandwich
from the glass plates.
In a preferred embodiment of the present invention, the cooling is
done by keeping the sandwich under pressure. As a matter of fact,
it has been observed that when the pressure existing during the
application of the high frequency is removed, defects such as
bubbles for example are obtained, and these defects may be
troublesome for certain applications.
We shall now refer to FIG. 2 which schematically illustrates an
example of a device for the implementation of the method for the
manufacture of high-density magnetic cards.
The magnetic card that is to be made is a high-density magnetic
card with a semi-rigid and dimensionally stable support bearing, on
the major part of at least one of its surfaces, a magnetic
recording medium formed by magnetic particles. The card support and
the magnetic medium itself should have a certain elasticity, i.e.
they should capable of recovering their original shape even after
repeated bending. The card should also be stable under heat to
withstand the usual environmental temperatures without deformation.
For these reasons, it has been chosen to make the card out of
polycarbonate, using several superimposed layers.
This card is therefore formed by a certain number of polycarbonate
sheets, ranging from two to ten, so that a sandwich is made with a
thickness of between 400 .mu.m and 1000 .mu.m, at least one of
these sheets bearing a magnetic layer. These polycarbonate sheets
may be made, for example, of LEXAN.TM. (manufactured by General
Electric Co.). The thickness of each sheet varies between 25 .mu.m
and 200 .mu.m. In fact, a higher weldability is observed with thin
layers than with thicker layers, one reason for this being that the
thinner the sheets, the greater will be the calorific losses by
conduction through the electrodes. This means that a greater
quantity of energy is needed to weld thin sheets.
The magnetic layer too is coated on a polycarbonate support having
a thickness that varies between 25 .mu.m and 75 .mu.m. The
thickness of the magnetic layer is about 5 .mu.m.
In the case of our example shown in FIG. 1, the card is made out of
four polycarbonate layers (7), (8), (9), (10) including the layer
supporting the magnetic layer.
By using roller feed devices, devices for the conveying, guiding
and centering of the tracks, these tracks are perfectly
superimposed on one another. By appropriate means (11 ), a first
cutting is then done to obtain a sandwich having the area needed to
make three cards (or more, depending on the dimensions of the
press). To keep the various layers perfectly superimposed, a first
welding (12) is done at each corner of the sandwich. This welding
may be done by ultrasonic means. The sandwich is then introduced
into a multi-station device comprising essentially a press that
enables the high frequency to be applied (13), a press provided
with a cooling circuit (14) and a device for cutting the individual
cards (15).
The sandwich first passes between the jaws of the press (13)
enabling high frequency to be applied. Each jaw of said press is
provided with a glass plate having a thickness of the order of 5
mm. The pressure is then set up, and finally the welding is carried
out by the application of the high frequency.
The following are the operating conditions in our embodiment:
Area of the sandwich to be welded: 200 mm.times.90 mm;
Press power: 6 KW;
HF frequency: 27.12 MHz;
Pressure on the sandwich: 100-200 N/cm.sup.2
Intensity provided by the generator: 1.5 A
HF application time : .apprxeq.10 s.
After this, the sandwich has to be cooled before it can be
released. For, the welding temperature has to be higher than the
glass transition temperature of the polycarbonate, and is generally
between 130.degree.-190.degree.. Owing to the planeity of the
surfaces required for such an application, this cooling is done by
maintaining the pressure used during the welding. This can be
carried out in the press used for the welding. About 30 s are
needed to come down to the desired temperature (about 30.degree.
C.-35.degree. C). In our embodiment, in order that the high
frequency press may be freed more quickly, there is provision for
another press (14), equipped with a cooling circuit, an appropriate
system being provided to transfer the sandwich under pressure into
said cooling press.
When the desired temperature is reached, the sandwich is removed
and sent to a cutting station (15) where the individual cards are
then made.
According to a preferred embodiment, before the sandwich is
introduced between the jaws of the high-frequency press, it is
passed into a pre-heating device. This may be, for example, a heat
press, the upper and lower jaws of which are taken to a temperature
of over 100.degree. C., preferably of the order of 150.degree. C.
In one embodiment, the duration of this pre-heating will be between
5 s and 10 s. In fact, this pre-heating will enable the
polycarbonate to react more swiftly at the next station when it is
subjected to the high frequency. For, below a certain temperature,
the dielectric losses of the material, enabling it to get heated
under the effect of the high frequency, are very small (for
polycarbonate, this temperature is of the order of 100.degree. C.)
and, above this temperature, they increase swiftly. This system of
thermal pre-heating, done at a first station (not shown) makes for
a substantial gain in time when the operation moves to the
high-frequency welding station.
This device with successive stations thus enables the fully
automated batch production of cards such as these.
Moreover, this welding device using glass plates has an additional
advantage for high density magnetic supports. In fact, for such
magnetic supports, it is necessary to have an extremely smooth and
even surface. For, the roughness and undulation of the surface of
the layer would prompt spacing effects at the reading or recording
heads in direct contact with said magnetic layer. These spacing
effects result in a modulation of the amplitude or fluctuation in
the level of the signals read or recorded, thus reducing the
signal-to-noise ratio obtained. Now, when the polycarbonate layers
are welded directly to metal surfaces, the peak-to-valley height
obtained is about 20 nm whereas, by interposing a polished glass
plate between the metal electrode and the magnetic layer, the
peak-to-valley height obtained is about 4 to 5 nm. We thus obtain
an improvement in the surface condition of magnetic layer. In the
prior art, this was obtained by an additional calendering
operation.
* * * * *